I really have to train my eye. I did not notice any tight spacing for the fake. Is there any reason for this? And is it coming from a special entity using 0.75em?

You probably don't notice because it's the right space for uppercase letters, but everything scaled down to 75%. With true smallcaps, however, only the letters are scaled down (sort of), but spaces remain the normal size. It's probably more noticeable in a text with mixed normal and smallcaps pieces. Make it more extreme: